First you need to know the speed of the electrons. Applying a voltage to a free moving electron will impart kinetic energy to it.

E = V * e = 20kV * 1.6*10^-19 = 3.2*10^-15

and 1/2 * m v^2 = E

so v = (E*2/ m)^0.5 = 8.4*10^7 m/s

Note that I didn't factor in relativity. I'll leave that for you to figure out.

Now you can pick a radius of curvature that you would like, the centripetal force needed for that curvature, and the magnetic field needed to achieve that force from Lorentz's force law and the speed of the electron in your experiment.